Compression technology like the H.26x (ITU-T Q6/16 VCEG) standard and MPEG (Moving Picture Experts Group)-y standard that compresses the amount of information of images using redundancy specific to images have widely been used for the purpose of efficiently transmitting or accumulating digital images. In Joint Model of Enhanced-Compression Video Coding as part of activity of MPEG4, international standards called H.264 and MPEG-4 Part 10 (Advanced Video Coding; AVC) capable of realizing a higher compression rate by incorporating new functions based on the H.26x standard have been laid down.
One important technology in these image encoding methods is the inter-frame prediction. In the inter-frame prediction, content of an image to be encoded is predicted by using a reference image and only a difference between the predicted image and the actual image is encoded. Compression of the amount of code is thereby realized. When an object moves greatly in a series of images, however, the difference between the predicted image and the actual image increases and a high compression rate cannot be achieved by a simple inter-frame prediction. Thus, an attempt is made to reduce a prediction error in the inter-frame prediction by recognizing motion of an object as a motion vector and making compensation for a pixel value in a region where the motion appears in accordance with the motion vector. Such a technique is called motion compensation.
In HEVC (High Efficiency Video Coding) HEVC whose standardization is under way as the next-generation image encoding method succeeding the H.264/AVC, each coding unit (CU) in an image is further divided into one or more prediction units (PU) and a motion vector can be set to each prediction unit. The size and shape of the prediction unit in HEVC is more varied than those of a block in H.264/AVC and the motion of an object can be reflected in motion compensation more correctly (see Non-Patent Literature 1 below). Non-Patent Literature 2 below proposes a technology that predicts a motion vector using a spatial correlation or temporal correlation of motion and encodes only a difference between the predicted motion vector and the actual motion vector to reduce the amount of code of the motion vector. Non-Patent Literature 3 below proposes to reduce the amount of code of motion information by merging blocks having common motion information of neighboring blocks in an image.
Another important technology in the aforementioned image coding method is scalable video coding (SVC). The scalable video coding is a technology that hierarchically encodes a layer transmitting a rough image signal and a layer transmitting a fine image signal. Typical attributes hierarchized in the scalable video coding mainly include the following three:                Space scalability: Spatial resolutions or image sizes are hierarchized.        Time scalability: Frame rates are hierarchized.        SNR (Signal to Noise Ratio) scalability: SN ratios are hierarchized.        
Further, though not yet adopted in the standard, the bit depth scalability and chroma format scalability are also discussed.